View image of A female Japanese macaque mounts another female Credit: Paul Vasey During the winter mating season, competition is fierce for access to female Japanese macaques. But it's not for the reason you might think. Males don't just have to compete with other males for access to females: That's because in some populations, homosexual behaviour among females is not only common, it's the norm. One female will mount another, then stimulate her genitals by rubbing them against the other female.
Some hold onto each other with their limbs using a "double foot clasp mount", while others sit on top of their mates in a sort of jockey-style position, says Paul Vasey of the University of Lethbridge in Alberta, Canada, who has been studying these macaques for over 20 years.
To our eyes these encounters look startlingly intimate. The females stare into each other's eyes while mating, which macaques hardly ever do outside of sexual contexts. The pairings can even last a whole week, mounting hundreds of times. When they're not mating, the females stay close together to sleep and groom, and defend each other from possible rivals.
That many humans are homosexual is well known but we also know the behaviour is extremely common across the animal kingdom, from insects to mammals. So what's really going on?
Can these animals actually be called homosexual? View image of Japanese macaques Macaca fuscata mating Credit: But for most of that time, the documented cases were largely seen as anomalies or curiosities. The turning point was Bruce Bagemihl's book Biological Exuberance , which outlined so many examples, from so many different species, that the topic moved to centre stage.
Since then, scientists have studied these behaviours systematically. On the face of it, homosexual behaviour by animals looks like a really bad idea Despite Bagemihl's roster of examples, homosexual behaviour still seems to be a rarity. We have probably missed some examples, as in many species males and females look pretty much alike. But while hundreds of species have been documented doing it on isolated occasions, only a handful have made it a habitual part of their lives, says Vasey.
To many, that isn't surprising. On the face of it, homosexual behaviour by animals looks like a really bad idea. Darwin's theory of evolution by natural selection implies that genes have to get themselves passed on to the next generation, or they will die out. Any genes that make an animal more likely to engage in same-sex matings would be less likely to get passed on than genes pushing for heterosexual pairings, so homosexuality ought to quickly die out.
But that evidently isn't what's happening. For some animals, homosexual behaviour isn't an occasional event — which we might put down to simple mistakes — but a regular thing. When Vasey first observed the females mounting each other, he was "blown away" by how often they did it. The females were simply seeking sexual pleasure "So many females of the group are engaging in this behaviour and there are males sitting around twiddling their thumbs," he says.
There is no way the behaviour can be evolutionarily irrelevant. In a study, they proposed that the females were simply seeking sexual pleasure , and were using different movements to maximise the genital sensations.
But for all the homosexual pairings the females indulge in, Vasey is clear that they are not truly homosexual. A female may engage in female-female mounting, but that doesn't mean she isn't interested in males. Females often mount males, apparently to encourage them to mate more. Once they had evolved this behaviour, it was easy for them to apply it to other females as well. View image of Red flour beetles Tribolium castaneum Credit: Take male fruit flies.
In their first 30 minutes of life, they will try to copulate with any other fly, male or female. After a while, they learn to recognise the smell of virgin females, and focus on them. The males are using homosexual behaviour as a roundabout way to fertilise more females This trial-and-error approach may look rather inefficient, but actually it is a good strategy, says David Featherstone of the University of Illinois at Chicago, US.
In the wild, flies in different habitats may have slightly different pheromone blends. Male flour beetles use a distinctly sneaky trick. They often mount each other, and go so far as depositing sperm. If the male carrying this sperm mates with a female later, the sperm might get transferred — so the male who produced it has fertilised a female without having to court her.
In both cases, the males are using homosexual behaviour as a roundabout way to fertilise more females. So it's clear how these behaviours could be favoured by evolution. But it's also clear that fruit flies and flour beetles are a long way from strictly homosexual. View image of Laysan albatrosses normally mate for life Credit: One such species is the Laysan albatross , which nests in Hawaii, US.
Among these huge birds, pairs are usually "married" for life. It takes two parents working together to rear a chick successfully, and doing so repeatedly means that the parents can hone their skills together. What's more, they rear chicks, fathered by males that are already in a committed pair but which sneak matings with one or both of the females.
Like male-female pairs, these female-female pairs can only rear one chick in a season. Same-sex coupling is a response to a shortage of males The female-female pairs are not as good at rearing chicks as female-male pairs, but are better than females that go it alone.
If she did not, she might manage to mate but would struggle to incubate her egg and find food. And once a female forms a pair-bond, the species' tendency towards monogamy means it becomes life-long. There is even a subtle advantage for the females. The system means that they can get their eggs fertilised by the fittest male of the group , and pass his desirable traits on to her offspring, even if he is already paired with another female. But once again, the female albatrosses are not inherently homosexual.
The Oahu population has a surplus of females as a result of immigration, so some females cannot find males to pair with. Studies of other birds suggest that same-sex coupling is a response to a shortage of males , and is much rarer if the sex ratio is equal. In other words, the female Laysan albatrosses probably wouldn't choose to pair with other females if there were enough males to go round.
So perhaps we've been looking in the wrong place for examples of homosexual animals. Given that human beings are known to be homosexual, maybe we should look at our closest relatives, the apes. Bonobo sex also cements social bonds Bonobos are often described as our "over-sexed" relatives.
They engage in an enormous amount of sex, so much so that it's often referred to as a "bonobo handshake", and that includes homosexual behaviour among both males and females.
Writing in Scientific American in , he described pairs of female bonobos rubbing their genitals together, and " emitting grins and squeals that probably reflect orgasmic experiences ". But bonobo sex also plays a deeper role: Junior bonobos may use sex to bond with more dominant group members, allowing them to climb the social ladder.
Males that have had a fight sometimes perform genital-to-genital touching, known as "penis fencing", as a way of reducing tension. More rarely, they also kiss, perform fellatio and massage each other's genitals.
Even the young comfort each other with hugs and sex. View image of Bonobos live in tight-knit social groups Credit: They don't show a consistent sexual orientation Just like humans can use sex to gain all sorts of advantages, so can animals. For instance, among bottlenose dolphins , both females and males display homosexual behaviour. This helps members of the group form strong social bonds.
But ultimately, all concerned will go on to have offspring with the opposite sex. All these species might be best described as "bisexual". Like the Japanese macaques and the fruit flies, they switch easily between same-sex and opposite-sex behaviours. They don't show a consistent sexual orientation. Only two species have been observed showing a same-sex preference for life, even when partners of the opposite sex are available. One is, of course, humans. The other is domestic sheep.
View image of A male domestic sheep Ovis aries Credit: In , neuroscientists found that these males had slightly different brains to the rest. A part of their brain called the hypothalamus, which is known to control the release of sex hormones, was smaller in the homosexual males than in the heterosexual males.
That is in line with a much-discussed study by the neuroscientist Simon LeVay. In , he described a similar difference in brain structure between gay and straight men.
How could this preference for other males be passed on to offspring? This seems quite different from all the other cases of homosexual behaviour, because it is hard to see how it could possibly benefit the males. How could this preference for other males be passed on to offspring, if the males do not reproduce?
The short answer is that it probably doesn't benefit the homosexual males themselves, but it might benefit their relatives, who may well carry the same genes and could pass them on. For that to happen, the genes that make some males homosexual would have to have another, useful effect in other sheep. LeVay suggests that the same gene that promotes homosexual behaviour in male sheep could also make females more fertile, or increase their desire to mate.
The female siblings of homosexual sheep could even produce more offspring than average. View image of Ewes everywhere, but some rams don't care Credit: Jacinta Lluch Valero, CC by 2. It's not clear whether the same thing happens in wild sheep, and if LeVay's explanation is right it probably doesn't. Domestic sheep have been carefully bred by farmers to produce females that reproduce as often as possible, which might have given rise to the homosexual males.